Currently, for example, asymmetrical a-BPDA is being prepared from o-xylene via an oxidative coupling reaction which essentially yields a mixture of 3,3′,4,4′-biphenyl dianhydride (s-BPDA) and a minor product (2-6%) of a-BPDA. Consequently, a-BPDA is being produced in limited quantity and therefore is not commercially available in sufficient amounts, despite an enormous interest in preparing polyimides using a-BPDA. This disclosure provides alternative and more efficient processes for producing polyimides derived from asymmetric 2,3,3′,4′-biphenyl dianhydride (a-BPDA), 2,3,3′,4′-benzophenone dianhydride (a-BTDA), 3,4′-methylenediphthalic anhydride (a-MDPA) and 3,4′-(hexafluoroisopropylidene)diphthalic anhydride (a-6FDA).
The prior art (U.S. Pat. No. 3,940,426, UBE Industries) process for making a-BPDA relies on oxidative coupling of o-xylene or o-phthalate with an organic acid salt of palladium under oxygen pressure to produce a mixture of symmetrical and asymmetrical intermediates which are oxidized and cyclodehydrated to form a mixture of a-BPDA and s-BPDA. This mixture requires the additional process of separating the two isomers.
U.S. Pat. No. 4,294,976 discloses a process for preparing a mixture of biphenyltetracarboxylic acids (3,3′,4,4′-isomer, 2,3,3′,4′-isomer and 2,2′,3,3′ isomer) via an oxidative coupling of either o-xylene or o-phthalate in the presence palladium catalyst followed by hydrolysis. The mixture of isomeric biphenyltetracarboxylic acids were then subjected to fractionally recrystallization to obtain the corresponding 2,3,3′,4′-(minor amount) and 3,3′,4,4′-(major amount) and minute amounts of 2,2′,3,3′-biphenyl dianhydrides.
U.S. Pat. No. 4,958,002 discloses a dehydration process to obtain 3,3′,4,4′-biphenyl dianhydride after the corresponding 3,3′,4,4′-biphenyltetracarboxylic acid was isolated from 2,3,3′4′-biphenyltetracarboxylic acid. U.S. Pat. No. 5,258,530 (Mitsibishi) describes a coupling reaction of phthalic anhydride to form a mixture of 2,3,3′,4′-(major) and 2,3,3′,4′-(minor) biphenyl dianhydrides. U.S. Patent Publication No. 0088120 A1 (2003) discloses a process for producing predominately 2,3,3′,4′-biphenyl dianhydride (a-BPDA) with a minor amount of 3,3′,4,4′-biphenyldianhydride (s-BPDA) using palladium and copper catalyst with bidentate ligand. These prior art processes all yield mixtures of asymmetrical dianhydrides together with symmetrical dianhydrides, which then requires the separation of these isomers. In comparison, this invention discloses asymmetrical coupling of dimethylphenylboronic acid with o-xylene derivatives to provide asymmetrical dianhydrides without contamination by the symmetrical dianhydrides.